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<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <div id="root"></div>
    <div id="model"></div>

    <script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script>
    <script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>
    <script src="https://unpkg.com/babel-standalone@6/babel.min.js"></script>
    <script type="text/babel">

        function ChildA() {
            return ReactDOM.createPortal((<div className="child-a">
                <ChildB />
            </div>), document.getElementById('model'))
            // return (<div className="child-a">
            //     <ChildB />
            // </div>)
        }


        function ChildB() {
            return (<div className="child-b">
                ChildB
            </div>)
        }

        ReactDOM.render(<ChildA />, document.getElementById('root'))
    </script>
</body>

</html>